When 16-year-old Colleen Walker went missing from Bowraville, a country town in northern New South Wales, police told her family she had probably "gone walkabout". They said the same when Colleen's cousin, four-year-old Evelyn Greenup, disappeared three weeks later. Then a third Aboriginal child, Clinton Speedy, 16, vanished – and a fortnight later the bodies began turning up in bushland.

That was in 1991, and 21 years later the families of Colleen, Evelyn and Clinton – who lived in the same street – are still waiting for justice. They have endured two police investigations, two trials, an inquest and numerous appeals to prosecutors and politicians. Now, finally, they have a glint of hope that the man they are convinced carried out the murders may be brought to account.

Jay Hart was a white labourer who – unusually in a town notorious for its racial divisions – hung around Bowraville's Aboriginal housing estate. According to witness testimony at a 2004 inquest, he supplied alcohol and drugs for parties and made sexual advances to young women and girls, including Colleen. He had a history of violence, the inquest heard, including towards one of his former partners. When each of the three children disappeared, Mr Hart was on the scene. Now 46, he has always denied involvement. He was acquitted of murdering Clinton in 1994. At the 2004 inquest, the coroner recommended that he stand trial for killing Evelyn, and he was acquitted of that murder in 2006. He was not charged with Colleen's murder. Her body was never found, although her clothes were discovered in a river.

Some families would have given up after the acquittals. But not the families of Colleen, Evelyn and Clinton. They campaigned for the state's double jeopardy laws, which prevented a suspect being tried twice for the same offence, to be overturned – and succeeded. They then lobbied the Director of Public Prosecutions and the former Attorney General to reopen the case – unsuccessfully, despite the police claiming to have found "new and compelling evidence" against Mr Hart. Now they have asked the new Attorney-General, Greg Smith, to refer the case to the Court of Criminal Appeal, which could order a retrial. "We don't want to get our hopes up too high after all this time," Clinton's sister-in-law, Leonie Duroux, said yesterday. "But it's all we've got to pin our hopes on."

Despite the passage of time, the families remain bitter about the mishandling of the original investigation, which may have led to evidence being missed and key witnesses not coming forward. They are bitter, too, about the length of time the case has dragged on – and they believe that, in contrast to other child murders, it has received little attention because the victims were black.

Bowraville, nestling in verdant hills near the coast, has always been racially fraught. Just a few decades ago, Aboriginal people could not get served in cafés and they had to enter the cinema via a side door. And as recently as 1990 the pubs were still segregated.

Detective Inspector Gary Jubelin, who led a second investigation and uncovered new evidence, said: "The families often ask me whether this matter would have been handled differently if the children were white, and it's a question I find very difficult to answer. Certainly they have every right to feel they've been let down by the criminal justice system."

Had it not been for Mr Jubelin's diligence and commitment – he took on the case in 1997 and is still involved 15 years on – it is unlikely it would have got this far. What drives him is what drove him from the start. "From a detective's point of view, it doesn't sit well with me that three children living in the same street can be murdered and no one has been brought to justice," he said. On the night Colleen Walker disappeared in September 1990, she had rebuffed advances by Mr Hart, the 2004 inquest heard. He was later seen following her home. Evelyn Greenup vanished from a house where Mr Hart had attended a party. Clinton Speedy went missing in February 1991 after going to Mr Hart's caravan with his girlfriend.

His body, and Evelyn's, were found in bushland outside town, a couple of miles apart.

Frustratingly, for the families and for police, no jury has yet heard evidence about all three murders. They hope that will happen if a retrial takes place. "It's very important to look at all three cases together because of the very strong links and similarities," said Oscar Schub, senior partner with a Sydney law firm assisting the families.

Mr Jubelin is reluctant to criticise the initial investigation, but says it may have been hampered by "communication difficulties" between police and the Aboriginal community. One key witness, for instance, did not come forward until the inquest in 2004. "He had significant information, and when he was asked why he didn't provide it back then, he said 'I'm a blackfella and I drink, why would anyone believe me?'"

Two years later, when Mr Hart stood trial for Evelyn's murder, Mr Jubelin realised that Aboriginal witnesses were at a disadvantage because of cultural factors. As well as being intimidated by the court setting, they avoided eye contact and were prone to long silences – habits common among Aboriginal people, but which could be misinterpreted as evasiveness.

Although more than two decades have elapsed, few Australians know much about the Bowraville case – unlike, for instance, that of the Beaumont children, Jane, Arnna and Grant, who disappeared from an Adelaide beach in 1966 and have never been found.

One police source said: "If they [the Bowraville children] had been three white kids from an affluent Sydney suburb, I'm sure more pressure would have been brought to bear to speed things up and make sure everything was done properly. But for whatever reason it didn't grab the attention of the community, and so it sort of slipped under the radar."

Ms Duroux, who is the widow of Clinton's late elder brother, Marbuck, said: "We feel terribly let down by the justice system, and have felt at times like we're banging our heads against a brick wall and that the government doesn't care, otherwise there would have been a result years ago.

"We just want to see justice done and get some closure for the families, but also for the kids – they were innocent – and that they didn't die in vain."